Corrosion resistant steels, known as stainless steels, have long been known and are presently available with a variety of properties. Austenitic stainless steels, which are those consisting substantially of a single austenite phase, possess the best combination of corrosion resistance and good mechanical properties, particularly at high temperatures. Austenitic stainless steels in the past have been steels in which chromium and nickel are the principal alloying agents. However, nickel is not an abundant metal, and the increased demand for it has increased its price and made its supply uncertain, particularly in critical times. Substitutes for nickel in the chromium-nickel austenitic stainless steels have long been sought. Recently the combined use of manganese, nitrogen and chromium in carefully balanced amounts has produced an austenitic stainless steel. This steel is described in U.S. patent application Ser. No. 251,637, filed May 8, 1972.
Although the use of nitrogen, manganese and chromium in these stainless steels has produced excellent products, the production of such steels has not been without difficulty. Nitrogen escapes from molten metal as it cools, and it tends to come out of solution during solidification to produce porous ingots which are substantially useless. Particularly in slow cooling articles such as large castings with large cross sections, maintaining high levels of nitrogen in the steel is difficult; and obtaining useful, non-porous castings is correspondingly difficult. Nitrogen is a hardener so that high nitrogen steels are difficult to work. For some purposes a heterogeneous ingot structure may be desired even though it is more vulnerable to attack. In such cases an austenitic surface on the heterogeneous metal may be desired.